1. Field of the Invention
The present invention relates generally to refractory materials and articles, and their methods of manufacture, and more particularly to refractory materials and articles with high resistance to molten metals, molten salts, and high temperature corrosive environments.
2. Description of Related Art
In many fields there is a need for highly refractory, chemically resistant, and thermodynamically stable materials and articles. Materials and articles with high melting points and with good mechanical and electrical properties are needed for use in thermochemical processing technologies incorporating materials in the form of molten metals, molten salts, reactive gases, and other corrosive chemicals. One particularly important area of application is crucibles, molds, and other containment vessels. Another important area is processing tools like stirring rods, transport tubes, temperature probes, bricks and mortar, and coated structures.
Materials and articles are required which can withstand high operating temperatures and can be exposed to a variety of difficult to handle materials, ranging from reactive metals like Ti, Zr, Hf, V, Nb, Ta, and Be, to lanthanide metals La to Lu, to actinide metals like U and Pu; as well as commonly used aggressive salts like alkali-halides (e.g., NaCl, KCl, LiCl) and alkaline earth-halides (e.g., CaCl.sub.2, CaF.sub.2). In addition, refractory materials and articles must withstand thermal cycling conditions, i.e., heating and cooling, without spalling or cracking; and must be able to be tailored to be good thermal and electrical conductors or thermal and electrical insulators.
Some of the problems associated with thermochemical processes are the result of high temperatures and very chemically reactive materials used for specific functions in the processes. Molten salts and metals are extremely aggressive at high temperatures and attack the molds or crucibles used to contain them in the processing vessel.
Metals like tungsten and tantalum have been used for crucibles, but the molten salts gradually erode them and molten metals can form alloys or intermetallic compounds with them. When the metals are corroded or no longer useable, they are discarded as waste. Magnesia (MgO) and yttria (Y.sub.2 O.sub.3) are examples of ceramic materials that have been used for crucibles, but these materials are either wetted by the melts or crack easily from thermal shock or stresses that build up at the interface between the melt and crucible, so they get limited use and are then discarded as waste.
Some coating technologies have yielded promising results, although the materials and architectures produced by these methods have inherent problems as well. Important examples of inherent problems are those associated with protective coatings that are either painted or plasma sprayed on crucibles and molds. These coatings have different thermal and chemical properties than their host; thus, property mismatches cause spalling or cracking of the protective coating, causing contamination of the melt, and allowing the host material to be exposed to the environment and attacked.
In the past, contractors to the US Department of Energy have utilized crucibles and molds painted with beryllia (BeO) and yttria (Y.sub.2 O.sub.3) for containing reactive metals like beryllium and uranium. These coatings crack and spall into the melts, allowing the molten metals to attack the host, and causing impurities to be trapped in the final metal products. Other technologies widely employed for processing reactive metals are arc-melting, electron-beam melting, and cold-wall induction melting. Under these conditions, the reactive molten metals are frozen, or solidified, quickly at the surface of the containment vessel, and thus are not allowed time to react with the container. However, these processes require large capital investments, are complicated, and have high operating costs. Further, an inherent problem with these technologies is that they do not allow for thorough mixing (or alloying) which may be desired or required for the final properties of the products.
U.S. Pat. No. 5,084,312 to Krikorian et al describes molten metal containment vessels with rare earth or rare earth like sulfide and oxysulfide coatings which inhibit wetting. Also described therein are conventional materials for containment vessels, including graphite, refractory metals, oxides, and fluorides. U.S. Pat. No. 4,363,995 to Crawford et al describes metal oxide or metal sulfide coatings. U.S. Pat. No. 4,876,725 to Furukawa et al describes high density sintered articles of silicon carbide. U.S. Pat. No. 3,890,140 to Asbury describes an aluminum titanate crucible for molten uranium.